The formation of amines for example by the catalytic hydrogenation of nitriles in the presence of excess ammonia is well known. However the conversion of linear or branched primary diamines to cyclic imines is a more difficult procedure, particularly since the starting amines readily form higher amine compounds such as, e.g., secondary amines and tertiary amines with the result that a considerable proportion of the starting amines are lost as polymeric products.
Most of the prior art employs the liquid phase. However, Raab (German Pat. No. 738,448 ) teaches the conversion of 1,6 diaminohexane to perhydroazepine. The reaction is carried out in the gas phase over catalysts such as chromium or vanadium oxides. Temperatures of 300.degree.-450.degree.C are taught. This temperature range is much higher than has been found to be necessary with the metal catalysts claimed herein. In fact none of the prior art combines the particular catalysts, employed under the conditions of temperature, vapor phase reaction and preference for a hydrogen diluent taught herein. As shown in the examples this unique combination leads to unexpectedly high selectivity to saturated cyclic secondary amines while repressing the formation of linear or branched secondary amines, tertiary amines and other higher polymers. When a liquid phase reaction is conducted, significant quantities of heavy, worthless, polymeric residues are formed. Other metals such as rhodium and palladium give much poorer selevtivity to the cyclic imine. The use of hydrogen as a preferred vapor phase diluent is also not anticipated in the prior art. Hydrogen surprisingly plays a unique role beyond that of a diluent in producing the superior selectivity to cyclic imine (i. e. saturated cyclic secondary amine) claimed herein.